500 



BOTANY 



well -developed protliallia ; in the latter plant the archegonia are reduced to 



elongated cells penetrating the tissue of the 

 nucellus. In G'netum, on the other hand, either 

 no prothallium is formed hut the embryo-sac 

 becomes tilled with protoplasm in which are 

 numerous nuclei, or a prothallium is developed 

 in the lower portion of the embryo-sac while 

 the upper portion contains protoplasm with 

 In t nuclei. Each of the two generative cells 

 from the pollen-tube fuses with a female nucleus. 

 Of all the fertilised cells resulting from the 

 penetration of a number of pollen-tubes to the 

 embryo-sac only one develops into an embryo 

 (Figs. 468, 469). 



Fossil Gymnosperms ( 9 ) 



In contrast to what was seen to be the . asr 

 for the Pteridophyta, Gymnosperms have not 

 yet been detected in Silurian strata. They 

 appear first in the Devonian, but are sparingly 

 represented and first form an important con- 

 stituent of the flora in the Carboniferous. From 

 the Cycadofilices. which possessed stems with 

 secondary thickening and fern-like foliage and 

 had been hitherto regarded as Pteridophyta, 

 OLIVER and SCOTT ( 9il ) have recently separated 

 the Pteridospermeae ; these may be briefly char- 

 acterised as fern-like spennaphyta. They have 

 si i own that the seeds of Lagenostoma Lomaxi, 

 \Vill, belong toLyyiuodtudron, oneof the Cycado- 

 filices with fern-like foliage, and that this plant must therefore be placed in the 

 Pteridospermeae. It is highly probable that the whole of the Lyginopterideae and 

 possibly of the Medulloseae also must be placed in this group. The structure of 

 the seed of Lagenostoma resembles that of Cycas, but, instead of the single thick in- 

 tegument of the latter, had a cupular investment to the seed on the outside of the 

 integument. In North America fragments of leaves referred to Cordaites have been 

 found. This genus belongs to a peculiar type confined to the Palaeozoic rocks. Owing 

 to the excellence of the preservation of their remains the morphology of the Cordaiteae 

 is as well known as that of the existing Gynuiospernis. They were lofty trees with 

 linear or broad and lobed leaves. Their flowers differ considerably from those of 

 recent Gymnosperms. An important fact as bearing on the phylogeny of the group 

 is the presence of a male prothallus as a small multicellular body. The ovules and 

 seeds show great structural agreement with those of the Cycadaceae. 



With the exception of some less common fossils (Cycadites, Dicranophyllum}, 

 the former of which is doubtfully referred to the Cycadinae, the latter more 

 probably to the Ginkgoineae, Cordaites is the most richly represented ty]ie of 

 Gymnosperm found in the Carboniferous rocks. Undoubted Cycadaceae make 

 their appearance in the lower Rothliegende. In later strata the genus Walchia, of 

 uncertain affinity, appears; the vegetative organs, which alone are known, recall 

 the lowest Coniferac (Araucariaceae). In the upper Rothlicgcndu undoubted 



V\t:. -ttiy.UiU'liuit lineman. Embryo-sac. 

 r, upper portion with free nuclei : y>. 

 lower portion filled with the tissue of 

 the prothallium ; ps, pollen-tube : . 

 two arrested embryo- sacs. (x (56, 

 after LOTSY.) 





